[LLVM][Casting.h] Add trivial self-cast

[bzcheeseman]

Summary

Casting from a type to itself should always be possible. Make this simple for all users, and add tests to ensure we keep being able to do this. Ref: https://reviews.llvm.org/D125543

[rriddle]

The test failures look real here, can you take a look? Looks like this doesn't properly handle the non-const case.

[bzcheeseman]

In D125590#3513022, @rriddle wrote:

The test failures look real here, can you take a look? Looks like this doesn't properly handle the non-const case.

Yeah agreed - I'll take a look when I get a chance.

[bzcheeseman]

Found a better way to do it (I think) that avoids yet another partial specialization of CastInfo to avoid future ambiguous partial specialization shenanigans.

llvm/include/llvm/Support/Casting.h

struct CastInfo<To, From, std::enable_if_t<!is_simple_type<From>::value>> {
   }
 
   static inline decltype(auto) doCastIfPossible(From &f) {
     return SimplifiedSelf::doCastIfPossible(
         simplify_type<From>::getSimplifiedValue(f));
   }
 };
 
+template <typename T> struct CastInfo<T, T> {
+  static bool isPossible(const T &t) { return true; }
+  static decltype(auto) doCast(const T &t) { return t; }
+  static decltype(auto) castFailed() { return T(nullptr); }
+  static decltype(auto) doCastIfPossible(const T &t) { return t; }
+};
+
 //===----------------------------------------------------------------------===//
 // Pre-specialized CastInfo
 //===----------------------------------------------------------------------===//
 
 /// Provide a CastInfo specialized for std::unique_ptr.
 template <typename To, typename From>
 struct CastInfo<To, std::unique_ptr<From>> : public UniquePtrCast<To, From> {};